1. Field of the Invention
This invention relates to an information signal processing apparatus for processing information signals.
2. Description of the Related Art
Known apparatuses designed for processing information signals include information signal recording/reproducting apparatuses which record information signals on recording media and reproduce the recorded information signals from the recording media.
Among the information signal recording/reproducing apparatuses, a video tape recorder or a still video camera is arranged, for example, to record an image signal on a magnetic recording medium after frequency-modulating the image signal and to restore the frequency-modulated image signal recorded on the medium to the original image signal by frequency-demodulating the signal.
In the video tape recorder or a still video camera, a temporary attenuation of amplitude, called a dropout, often takes place in the signal reproduced from the magnetic recording medium at the time of reproduction due to various causes such as a foul or clogged state of a magnetic head and dust sticking to or scars left in the magnetic recording medium. Then, the demodulated signal waveform of the image signal obtained by frequency-demodulating the reproduced signal which has the dropout becomes a signal waveform which resembles a white noise waveform and greatly degrades the picture quality of the restored image signal, because the normal demodulating action of a frequency demodulator is hindered by the occurrence of the dropout. In view of this, the conventional video tape recorder (VTR) or still video camera has been provided with a dropout compensation circuit which is arranged as shown in FIG. 1 to improve the picture quality by making compensation for the dropout occurring in the image signal reproduced.
Referring to FIG. 1, a weak reproduced signal obtained from a magnetic recording medium by a magnetic head which is not shown is amplified by a preamplifier 26. The reproduced signal thus amplified is supplied to a recorded/unrecorded detector 27, a frequency demodulator 28 and a dropout detector 29.
The recorded/unrecorded detector 27 is arranged to find if any image signal is recorded or not recorded in each track on the magnetic recording medium currently under the reproducing action of the magnetic head on the basis of the level of the signal reproduced. A detection signal which indicates the result of detection is outputted from an output terminal 34.
Further, if the detection signal outputted from the output terminal 34 indicates the presence of the record of the image signal in a track on the magnetic recording medium, a tracking control mechanism which is not shown performs tracking control in such a way as to cause the magnetic head to trace an optimum position of the track on the magnetic recording medium. The recorded/unrecorded detector 27 has its time constant set at a relatively long period of time of the order of several milliseconds for detection of any recorded signal. The length of this period corresponds to some fraction of one vertical scanning period of the TV signal.
A dropout compensation process is performed after the detection of recorded or unrecorded state of the image signal and the tracking control are performed in the manner described above.
At the frequency demodulator 28, the reproduced signal supplied from the preamplifier 26 is first frequency-demodulated. The demodulated image signal outputted from the frequency demodulator 28 is supplied as it is to one terminal A of an analog switch 32. The demodulated image signal outputted from the frequency demodulator 28 is supplied also to a 1-H delay line 31 which is composed of such elements as a CCD (charge-coupled device), etc., to be delayed as much as one H (H: one horizontal scanning period of a TV signal). The one-H delayed demodulated image signal is supplied to another terminal B of the analog switch 32. The analog switch 32 is normally in connection with the terminal A. The switch 32 is, however, connected to the other terminal B upon detection by the dropout detector 29 of occurrence of a dropout in the reproduced signal supplied from the preamplifier 26. With the switch 32 connected to the terminal B, the frequency-demodulated reproduced signal which has been delayed by one H period and has no dropout is outputted in place of the frequency-demodulated signal having a dropout. A dropout compensating action is carried out in this manner.
The details of the dropout detector 29 of FIG. 1 are described below with reference to FIG. 2:
As shown in FIG. 2, the dropout detector 29 is composed of an automatic gain adjustment circuit 37, an envelope detection circuit 38 and a pulse shaping circuit 39. Generally, the reproduced signal supplied from the preamplifier 26 of FIG. 1 has some fluctuations or waviness in amplitude due to uneven contact of a magnetic head with a magnetic recording medium or the magnetic orientation characteristic, etc., of the magnetic recording medium.
Therefore, in detecting the dropout, the adverse influence of the amplitude fluctuations is eliminated by causing the reproduced signal supplied from the preamplifier 26 of FIG. 1 to pass through the automatic gain adjustment circuit 37 of FIG. 2. After that, dropout detection is made at the envelope detection circuit 38 by following minute dropout parts at short time constant of the order of several hundred nanoseconds. A detection signal which is thus outputted from the envelope detection circuit 38 is supplied to the pulse shaping circuit 39. At the pulse shaping circuit 39, the pulse width and the time base of the detection signal are corrected in such a way as to make them correspond to the white noise generated in the demodulated signal outputted from the above-stated frequency demodulator 28 due to the occurrence of dropout. After the correction, the detection signal is supplied to the above-stated analog switch 32 as dropout detection pulses. The analog switch 32 is connected to the terminal B for the period during which the dropout is occurring in the reproduced signal. By this, the demodulated signal which has become the white noise due to the occurrence of the dropout is replaced with the demodulated signal which is obtained one horizontal scanning (H) period before. As a result, a dropout-compensated image signal is outputted from the output terminal 35 of FIG. 1.
In the dropout detector which is used for the dropout compensation process described above, the automatic gain adjustment circuit is deemed to be indispensable for the input stage of the circuit arrangement. Generally the automatic gain adjustment circuit consists of a detector for detecting the amplitude of the input signal and a variable gain amplifier which is controlled by the output of the detector. These circuits require many elements and are, therefore, not suited for discrete circuit arrangement.
In view of this, it has been practiced to have the dropout detector including the above-stated automatic gain adjustment circuit arranged as a custom-made IC or to utilize a multi-function signal processing IC which already includes a dropout detector and is adapted for a video tape recorder or the like.
However, with the dropout detector arranged to be a custom-made IC, it results in a great cost increase in the event of manufacture in a small quantity. In the case of use of a dropout detector which is supplied in a state of being included in a multi-function IC, if the functions of the IC are to be efficiently utilized, signal processing methods other than the dropout detecting method must be also arranged to comply with the multi-function IC. This imposes restrictions on design work.